The present invention relates generally to voltage controlled attenuators and, more paticularly, to a voltage controlled attenuator which is especially useful in association with an electronic musical instrument.
Voltage controlled attenuators, frequently referred to in the art as voltage controlled amplifiers, are typically employed in electronic musical instruments as keying circuits wherein an audio signal is selectively attenuated in accordance with an input control voltage. The audio signal, a sine wave for example, is applied to the data input of the attenuator while the control voltage, usually in the form of a desired envelope signal, is coupled to the control input of the attenuator. An output signal is thereby produced consisting of the input audio signal amplitude modulated in accordance with the control voltage envelope. Voltage controlled attenuators are also sometimes used in electronic musical instruments to control the operation of a voltage controlled oscillator. For example, a "trill" sound is typically produced by suitably controlling the level of a square wave signal coupled to the control input of a voltage controlled oscillator.
Various techniques have been used in the past to implement voltage controlled attenuator designs. Such prior art techniques include the use of photocells, operational transconductance amplifiers and various forms of multiplying circuits, both digital and analog. The performance of these prior art circuits, while satisfactory in certain respects, is somewhat deficient in others. In this regard, a voltage controlled attenuator, especially one having application in an electronic musical instrument, is preferably characterized by a number of important qualities. Initially, the attenuator should have a high degree of environmental stability. Devices incorporating photocells and operational transconductance amplifiers are quite temperature sensitive and therefore have difficulty in providing the required degree of environmental stability. The attenuator should also provide extremely accurate levels of attenuation and have a strictly linear response over a relatively wide dynamic range of attenuation settings and input frequencies. Another highly desirable attribute of a voltage controlled attenuator resides in the provision of an exponential or logarithmic transfer function allowing for a relatively wide control capability using only a small differential control voltage. Such an exponential control transfer function also results in a constant percentage accuracy wherein higher resolution attenuation is achieved at low voltage control signal levels and somewhat lower resolution is achieved for higher control voltage signal levels. Yet another desirable characteristic of voltage controlled attenuator design is the provision of a relatively flat bandwidth from DC to some desired frequency.
One aspect of prior art voltage controlled attenuators which contributes to the inability of such devices to meet or satisfy the above criteria involves the rather extensive use of active electronic components. The voltage controlled attenuator of the present invention, on the other hand, employs techniques which may be implemented using only passive components and switching circuits, thereby enabling the aforementioned design criteria to be largely satisfied by a relatively simple and inexpensive circuit which is easily implemented in in integrated circuit form.